1. Field of the Invention
This invention relates to a motion compensation system to reduce the effects of relative motion between dynamically moving ramps. Specifically, the motion compensation system allows a ramp undergoing dynamic motion to safely interface with a relatively motionless structure.
2. Description of the Related Art
Both the commercial and military shipping industries have found that roll-on/roll-off (RORO) ships provide a fast and cost effective means for transporting cargo. Commercially, RORO ships are used as auto ferries due to the ease and speed with which passengers can rapidly self-load their cars onto the ship. In military applications, a RORO ship also allows the deployment of cargo and material without the use of extensive on-shore infrastructure, which gives RORO ships the capability to be used in relatively primitive conditions.
One of the downsides of the RORO ship construction is that, in high seas, they are difficult to load and unload. The key to the success of these ships requires that the ship come with a solid ramp that is sufficiently sturdy to hold the weight of automobiles and the like. Since these ramps are solid, they cannot flex or bend. While this inflexibility is not a major problem in low seas when the wave action is largely absorbed by the inertia of the ship itself, this inflexibility can be a major problem in high seas where the ship is undergoing large scale lateral, vertical, and rolling motions. In these situations, the RORO ramp will no longer cleanly interface with the pier or floating storage facility, making it unsafe to load and unload cargo.
Previous efforts to design loading and unloading systems for RORO ships did not adequately account for the high seas motions that RORO ships undergo. While these systems might allow relative motion between two platforms, they did not allow the RORO ship's ramp to move and roil at the point where it is most needed: where the ship ramp meets the pier. For example, in U.S. Pat. No. 4,441,449 to Biaggi, the invention is of a floating pier that was free to pivot about an attachment point on land. While this pier was able to move, this pier did not account for the relative rolling and sliding motions that occurs at the interface between the pier and the ship's ramp during high seas. The same is true in another floating pier concept shown in U.S. Pat. No. 5,823,715 to Murdoch et al. In these situations, the ship would be unable to load or unload safely since the RORO ship's ramp would no longer securely communicate with the dock.
In a variation on this concept, U.S. Pat. No. 5,359,746 to Kane et al. discloses a ramp junction device that accounts for the various motions between a movable platform, a movable ramp, and a fixed platform. However, this system requires that the ramp account for the various motions through a connection point at each end of the ramp. In this, and in other similar systems such as those disclosed in U.S. Pat. Nos. 4,337,545 to Rose et al., 4,169,296 to Wipkink et al., 4,003,473 to Ryan, and 4,590,634 to Williams, the essence of the solution is to provide complex connection points at the edges of a solid ramp to account for the relative motion of two maritime platforms. These complex connections often require special hardware and on-shore infrastructure that are not always available and/or practicable.